The link below explains that it is a flat tax of 15% for Non Thailand Tax residents & taxed as personal income for Thailand Tax residents. For building they tax 70% of the rent as 30% is allowed for maintenance etc. More than 30% can be claimed if you got supporting documents.
So if your wife is a non working Thai under the age of 65 then all you get if you file your tax as a coupe is you can claim her 60,000 THB tax allowance but you can both file as singles. So she has 60,000 tax allowance. Then the first 150,000 is 0% tax. So she pays no tax paid up to 210,000 & as only 70% of the rent is taxable that means you can get 300,000 (25,000/month) in rent and pay no tax. The next 150,000 (70% of rent = 214,000 full rent) you only pay 5% tax which is 7,500 on the full 150,000. So for rent of 514,000 (42,800/month) you paying 7,500 a year tax. After that it goes up as it goes through the higher tax rate bands.
https://phuketrealtor.com/blog/tax-on-rental-income-in-thailand-what-should-you-know